The drilling phase of an oil or gas well consists fundamentally of drilling a hole down into a subterranean formation by means of a bit on the end of the drill string. As the drilling continues, the wellbore is lined or “cased” in sections using tubular casing material. Throughout the drilling operation, drilling fluids or muds are circulated through the well to cool and lubricate the bit and flush the cuttings up from the bottom of the wellbore. The mud is circulated continuously from a pumping unit at the surface down through the inside of the drill string, out through the end of the drill string or bit at the bottom of the well, and then back up the annulus between the inside of the wellbore or casing and the outside of the drill string.
A common problem encountered during drilling is the excessive loss of drilling fluids out into voids in the formation. This fluid loss is often referred to as “lost circulation.” “Voids” refers to spaces in the formation, usually not formed by the drill bit, such as fractures, fracture networks, caverns, cracks, fissures, vugs, cavities, washouts, cobble packs, unconsolidated sands, and the like, which may extend from an opening in the wellbore wall deep into the adjacent formation. Lost circulation is costly because the lost drilling mud must be replenished constantly to maintain an adequate volume of fluid in the well. In addition, lost circulation may result in destabilization of permeable formations and damage to the wellbore. “Fluid loss control” and similar expressions used herein refer to efforts to reduce or avoid excessive fluid loss, such as lost circulation.
Various materials and methods have been used to deal with lost circulation. Among the materials that have been used to control lost circulation are shredded automobile tires, nylon rope fibers, sawdust, wood chips, mica flakes, cottonseed hulls, ground nutshells, burlap bags, shredded diapers, and various gels or slurries, such as hydraulic cement. While these materials have met with some success, there remains a need for a lost circulation control technology that is cost effective, easy to use, long-lasting, and environmentally friendly.